I. Field of the Invention
This invention relates generally to a method for preparing stranded wires to facilitate their later use in connecting electrical circuit components, and more specifically to a method for forming a small, solid, generally spherical bead at the tip end of the conductor.
II. Description of the Prior Art
In making connections to electrical circuit components, it is often advantageous to use a brazing step wherein an electrical conductor is joined to a conductive pad area on the component as by spot welding or brazing. Where pad or terminal areas on the component are closely spaced, it is important that the bonding operation not lead to short circuiting of adjacent terminal pads. This problem is particularly acute where multi-strand electrical wire is being joined to a pad area. It is found that when the head of the spot welder is brought into contact with the multi-stranded wires to be joined to the pad, the strands are squeezed and tend to spread out or fray. These frayed ends may then bridge the gap between two adjacent pad or terminal areas on a substrate to which the wire is being joined.
The foregoing problem is further aggravated when the conductor being joined to a pad area is a multi-stranded phosphor-bronze alloy. While phosphor-bronze affords the advantage of being quite resilient and able to withstand repeated flexures, it presents a significant problem when attempts are made to weld or otherwise affix a stripped end of such multi-standed conductors to a terminal pad. The individual strands of phosphor-bronze conductors tend to be somewhat stiff and will fan out when a welding tip is pressed against the conductor during an attachment step. Thus, where the terminals on the electrical device to which the wire is being attached are closely spaced, this fanning out of the ends of the individual strands in a stranded conductor can result in short circuits between adjacent terminals.
Where a multi-stranded conductor is to be soldered to a terminal pad, it has been common practice to first dip the conductor in liquid solder to "tin" the tip. This tends to render the tip end inflexible and subject to fracture after repeated flexures. For this reason, as well as for others, fusion bonding has found widespread use.
The method of the present invention is designed to obviate problems occasioned by fraying of the stranded wires. Before the attachment step is performed, the wires to be joined to the electrical device are pre-treated whereby a small cohesive bead is formed at the tip of the multi-stranded conductor. This bead holds all of the individual strands together with no frayed ends extending therefrom. In carrying out the invention a portion of the insulation on the multi-stranded conductor is stripped from the ends thereof to be beaded and this stripped portion is gripped in a holding fixture. Next, the exposed end of the stranded wire to be treated is moved into contact with a tungsten electrode and this electrode is then moved away from the tip of the stranded wire by a predetermined gap distance. Next, a high voltage, high frequency field is established across the gap to form a plasma breakdown such that a charged capacitor may be made to discharge through the electrode, the gap and the wire being treated. This discharge current is sufficient to cause the individual strands at the tip of the conductor to be melted. Upon cooling, a tiny, generally spherical bead of conductive metal is formed at the tip of the wire without any projecting, frayed ends exiting from the bead.
Now, when the wire so pre-treated is to be joined to a conductive pad area on a circuit component or the like in a brazing step, relatively high pressures can be applied forcing the bead against the terminal pad without fear that the application of this pressure will cause fraying of the strands of the electrical conductor being joined to the pad.
It has also been found expedient to introduce an inert gas into the gap between the electrode and the wire's tip prior to the application of the discharge current to prevent oxidation of the metal comprising the wire. This prevents splattering of the metal particles and leads to the improved formation of a small spherical bead.